1. Field of Invention
The present invention relates to optical fiber collimators, and more particularly, to the coupling of optical radiation between optical fibers and optical devices.
2. Description of the Background Art
Micro-optic fiber devices are widely used in applications such as: optical communication systems, fiber optical source systems, and fiber sensor systems. Micro-optic fiber devices typically comprise optical management elements disposed between an input optical fiber and an output optical fiber. This configuration provides a means by which parameters such as light polarization status, signal power level, optical spectral, optical temporal, and optical beam direction can be modified. The optical fiber device typically functions to couple optical radiation from the input optical fiber for propagation through an optical management element, for example, and also functions to couple the optical radiation from the optical management element into the output optical fiber with maximum efficiency.
FIG. 1 shows a conventional small-core optical collimator 10 comprising an optical fiber 11 with a single-mode core 13, as may be used in an optical switching network (not shown). Optical signals in a light beam 19 may be output from the optical fiber 11 and coupled to a C-lens 15. In the example shown, the mode field of the optical fiber 11 may have a constant diameter of about 4 μm or less. The numerical aperture (NA) at an end face 17 of the optical fiber 11 may be about 0.2 or higher. A margin ray 21 passes near the C-lens 15 and converges at a focus point 23 on an optical axis 25. A paraxial ray 27 converges at a focus point 29 on the optical axis 25. Accordingly, the small-core fiber collimator 10 produces aberration as a consequence of the focus point 23 being spatially displaced from the focus point 29.
FIG. 2 shows the conventional small-core optical collimator 10 in optical communication with a second conventional small-core optical collimator 30 that includes a second C-lens 31 and a second optical fiber 33 having a second single-mode core 35. In the configuration shown, the paraxial ray 27 is focused into the second optical fiber 33, which results in a correspondingly low insertion loss. The margin ray 21, however, divergences to produce a larger beam spot at the end of the second optical fiber 33, which results in a correspondingly higher insertion loss.
An alternative configuration of fiber collimators, that includes a fiber pigtail and a collimator lens, may be used to reduce such insertion loss. In such configurations, a large parallel optical beam is transmitted through optical management elements and focused into the output fiber. Commercially-available fiber collimators fabricated from SMF28 fiber, such as may be used in C-band applications, have a comparatively low insertion loss of about 0.2 dB. However, some commercial applications require the use of optical fiber having core diameters of about 4 μm or less. These small-core optical fibers include, for example, HI980, HI1060 flexcore, and RC1550, which may be used in either an erbium-doped fiber amplifier (EDFA) pumping application or in a miniature sensor system. A shortcoming of this configuration is that fiber collimators fabricated using such small-core fibers may produce unacceptably high insertion losses of about 0.7 dB. The major reason for the relatively high insertion loss typical of small-core fiber collimators is primarily due to spherical aberration of the transmitted optical signal.
The small-core fiber must have a core with a relatively high index of refraction, i.e., a large numerical aperture (NA), if single-mode operation is to be maintained. However, when the NA of the small-core fiber is 0.2 or higher, and the small-core fiber is coupled to a C-lens, the diameter of light beam collimated by the C-lens may become unacceptably large and may produce non-paraxial rays which then cannot be efficiently coupled into a small core fiber at an output side. In other words, a large NA means that the light beam emitting from a small-core input fiber will exhibit a large divergence angle and, accordingly, will form non-paraxial rays in the circle of least confusion in the focal plane. Correspondingly high insertion loss may thus occur when the minimum circle diameter is larger than the small core fiber mode field diameter (MFD). The smaller the fiber core, the greater are the insertion losses that may result.
In the present state of the art, there continues to be interest in further miniaturization of optical fiber devices. The physical reduction of optical device elements requires interaction with a correspondingly smaller optical beam diameter. However, conventional manufacturing methods are not able to routinely produce either the requisite curvature radii for a C-lens collimator or the index profile for a GRIN lens collimator for such small optical beams. For example, while a miniature optical device may require a beam diameter of only about 80 to 150 μm, the minimum C-lens beam diameter attainable by conventional manufacturing methods may be about 230 μm for an SMF28 fiber application.
Certain methods practiced in the prior art are directed to the fabrication of fiber-like GRIN lenses. For example, U.S. Pat. No. 6,542,665 “GRIN fiber lenses” issued to Reed et al. discloses a GRIN fiber lens whose core refractive index radial profile has a radial second derivative specified to be a function of the refractive index on the axis of the fiber lens. U.S. Pat. No. 6,847,770 “Lens function including optical fiber and method of producing the same” issued to Kittaka et al. discloses a lens function comprising a gradient index optical fiber joined to an end surface of a step index optical fiber.
However, such methods have not found widespread application because of the complexity of the required manufacturing processes, such as ion exchange, and the associated costs. Additionally, while the use of a gradient index fiber lens seems like a viable solution, the resulting working distance is only about 1 to 2 mm. This relatively small working distance limits the usefulness of GRIN fiber lenses to isolator applications such as described in, for example, U.S. Pat. No. 6,643,428, “Optical fiber collimator and method for fabricating the same,” issued to Chang, which discloses an optical fiber collimator comprising a single-mode fiber spliced to a graded-index multi-mode fiber of specified length.
Moreover, a fiber device may typically be required to operate over a temperature range of from −5 to 70° C. The resulting thermal effects on the metal and epoxy components found in the device may result in the collimator beam shifting slightly, and may produce even greater insertion loss. Hence, temperature dependence loss (TDL) can become a significant factor in small-core fiber devices. In addition, the power demand for certain optical devices continues to increase for newer components. For example, the typical power handling requirement has increased from about 500 mW to 2 W, or more. It has been shown that most fiber collimators, especially small-core fiber collimators, tend to fail under this power level.
In one conventional approach to solve the problems discussed above, a thermal diffusion process was used to heat the input fiber so as to expand the fiber core. U.S. Pat. No. 5,757,993 “Method and optical system for passing light between an optical fiber and GRN lens” issued to Abe discloses a core expanded fiber produced by locally diffusing dopant contained in the core member using a thermally expanded core technique. However, this core-expansion method can be very time consuming and costly. For example, the method may require a heat treatment process of about 20 minutes and a temperature of approximately 1700° C. in order to expand a fiber core from 4 μm to 10 μm. To expand the fiber core to 30 μm, for example, may require a heating process operating from fifty to sixty minutes for completion. Moreover, since the heat source, typically a flame or micro quartz oven, may not provide a stable temperature, the repeatability of the core expansion process may be unacceptably low. Hence, a conventional fiber core-expansion method may not be adaptable to mass production.
What is needed is a device and method for quickly and efficiently expanding output from a small core optical fiber, that lends itself to mass production, provides a high yield, and has a low fabrication cost. It is thus an object of the present invention to provide such a method of low-loss coupling to a small core optical fiber via a C-lens which is also applicable to miniature optical devices. It is also an object of the present invention to provide such low-loss coupling devices having thermal stability and increased power handling capacity.